If the domain alias is hosted on a SecureHost server:

1. Log in to WHMCS and go to DNS Manager.
2. Click on Add Zone and enter the alias url in the Zone Name box that appears and then click Add Zone.
3. Then click Edit Zone. 
4. Make sure that the nameservers are SecureHost nameservers.
5. Set the TXT field to v=spf1 a mx ~all
6. Make sure the A record IP address is correct and click save.